Chapter 9 Mass Spec 1

7/24/2019 Chapter 9 Mass Spec 1

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INSTRUMENTAL ANALYSIS

CHEM 4811

CHAPTER 9

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natura sciences

!ayton state uni"ersity


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CHAPTER 9

MASS SPECTROMETRY I

PRINCIPLES AND APPLICATIONS


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PRINCIPLES

Technique involves

- Creating gas phase ions from the analyte atoms or molecules

- Separating the ions according to their mass-to-charge ratio (m/z)

- Measuring the abundance of the ions


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PRINCIPLES

Technique can be used for

- ualitative and quantitative analysis

- !roviding information about the mass of atoms and molecules

- Molecular structure determination (organic " inorganic)

- #dentification and characterization of materials


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PRINCIPLES

- #nstrument is mass spectrometer

- Separates gas phase ionized atoms$ molecules$ and fragments

of molecules

- Separation is based on the difference in mass-to-charge ratio (m/z)

m % unified atomic mass units (u)

& dalton ('a) % & u % &*+, . &,-0g

z % charge on the ion (may be positive or negative)


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PRINCIPLES

- 1nalyte molecule can undergo electron ionization

M 2 e- 3 M42 2 e-

- M42is the ionized analyte molecule called molecular ion

- 5adical cation is formed by the loss of one electron

- Computer algorithms are used to deconvolute m/z values of multiplycharged ions into the equivalent mass of singly charged ion

- !ermits easy determination of molecular 6eight of analyte


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THE MASS SPECTRUM

- 1 plot of relative abundance vs m/z

- The most abundant pea0 is 0no6n as the base pea0

- The base pea0 is scaled to &,,

- Spectrum sho6s fragmentation patterns

- The m/z values and the fragmentation pattern are used to determinethe molecular 6eight and structure of organic compounds

- !rovides the accurate mass of a given isotope not the

6eighted average


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RESOLVING POWER

- The ability of a mass spectrometer to separate ions of t6o

different m/z values

- 5esolving po6er % M/7M

- M % mass of one singly charged ion

- 7M % difference in mass bet6een M and the ne.t m/z value

- The resolving po6er of ions in the ,, range % ,,

- The resolving po6er of ions in the &,, range % &,,


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RESOLVING POWER

- T6o methods used to calculate 7M

- 8ull 6idth at half ma.imum (89:M) % 7M

- &,; valley (overlap should not be < &,;)

RESOLUTION

- The value of 7M at a given M

- =.pressed in ppm


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INSTRUMENTATION

Main componn!" o# !$ ma"" "pc!%om!%

- Sample input system

- #onization source

- Mass analyzer

- 'etector

- >acuum pumps

- Computer based data acquisition and processing system


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SAMPLE INPUT METHODS

Ga" E&pan"ion

- ?seful for gas samples and liquids 6ith sufficiently high

vapor pressures

- The gas or vapor e.pands into an evacuated and heated vessel

- Sample lea0s through holes in a gold foil seal into the ionization

source (termed molecular lea0 inlet)

- !ressure in ionization is maintained at &,-@ &,-Atorr


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Di%c! In"%!ion P%o'

- 8or liquids 6ith high boiling points and solids 6ith sufficiently

high vapor pressure

- The probe (6ith the sample in a glass capillary at the tip) is

inserted into the ionization source

- The probe is electrically heated to vaporize the sample

- This method has a problem 6ith contamination


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Di%c! E&po"(% P%o'

- Sample is first dissolved in a solvent

- 1 drop of solution is placed at the rounded glass tip of the probe

- Solvent evaporates leaving a thin film of sample

- The tip is inserted into the ionization source and heated tovaporize sample

- Bess li0ely to be contaminated


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C$%oma!o)%ap$* an+ E,c!%op$o%"i" S*"!m"

- Chromatographic instruments are used to separate mi.tures of

gases and liquids

- Separated components are introduced into a mass spectrometer

for detection

- The C-MS system

- BC-MS system is used for nonvolatile organic compounds

- Capillary electrophoresis (C=) can also be coupled to MS


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IONI-ATION SOURCES

E,c!%on Ioni.a!ion /EI0

- Commonly used for analysis of organic samples

- =lectrons are emitted from a heated tungsten filament cathode

- =lectrons are accelerated to6ards the anode 6ith a potential

of about *, @ &,, >

- =lectrons meet at right angles 6ith the sample molecules

- #nteraction 6ith the high energy electrons causes ionization of

sample molecules and fragmentation into smaller ions


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IONI-ATION SOURCES


- 5eferred to as hard ionization source due to the high energy

=# source

- #ons are accelerated into the mass analyzer by an accelerating

voltage of D &,+>

- Eoth negative and positive ions are formed by =#

- Fegative ions form from molecules containing acid groups or

electronegative atoms


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IONI-ATION SOURCES


- Collision bet6een ions and molecules may also result in ions

6ith higher m/z values than the molecular ion

1n e.ample is the (M2&) pea0

- 5eaction bet6een analyte molecule and :2to form M:2

or (M2:)2in 6hich charge equals a2&

- Bo6 pressure in the ionization source minimizes reaction

bet6een ions and molecules


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IONI-ATION SOURCES

C$mica, Ioni.a!ion /CI0

- 1 large e.cess of reagent gas (&,,, @ &,,,, times) is introduced

into the ionization region

- !ressures in source are typically higher than =#

- =lectrons are allo6ed to bombard the gas-sample mi.ture

=.amples of reagent gas

- Methane$ ammonia$ isobutane


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IONI-ATION SOURCES


- 5eagent gases are much more li0ely ionized by the electrons

than sample due to large e.cess

- Sample molecules are subsequently ionized by collision 6ith

ionized reagent gas molecules

- Considered soft ionization source

- Bess fragmentation and molecular ion is much more abundant

- Combination of C# and =# spectra provide good interpretation


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IONI-ATION SOURCES


- 8or methane reagent gas

electrons6ithninteractiouponformedareC:andC:G+

++

++ ++ G*++ C:C:C:C:

*+G ::CC:C: ++ ++

!roton transfer occurs 6hen sample molecules collide 6ith

++** :CandC:


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IONI-ATION SOURCES


+* C:M:C:M ++ ++

The follo6ing may occur if analyte is a saturated :C

+* :CM::CM ++ ++

+* :C::)-(MC:M +++

++

* :C:)-(M:CM ++ ++

H)(Mm/z6ith):C(M:CM ** +=++ ++


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IONI-ATION SOURCES

A!mo"p$%ic P%""(% Ioni.a!ion /API0 So(%c"

- T6o maIor types

=lectrospray #onization (=S#)

and1tmospheric !ressure Chemical #onization (1!C#)

- Jperate at atmospheric pressure

- Modified version of =S# is the #on Spray Source

- ?sed for mi.tures of nonvolatile high molecular 6eight compounds


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IONI-ATION SOURCES

A!mo"p$%ic P%""(% Ioni.a!ion /API0 So(%c"

1pplications

- !harmaceutical chemistry

- Eiochemistry- Clinical biomonitoring

=lectrospray- 8ine spray of positively or negatively charged droplets


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IONI-ATION SOURCES

D"o%p!ion Ioni.a!ion

- 8or direct ionization of solids

- =.cellent tool for analysis of large molecules

- Solid samples are placed on a support and then bombarded

6ith ions or photons

- 'ifferent types are available


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IONI-ATION SOURCES


'esorption Chemical #onization

- ?sed for nonvolatile compounds

- Sample is directly introduced into the chemical ionization

source on a tungsten or rhenium 6ire

Secondary #on Mass Spectrometry (S#MS)

- 8or surface analysis

- 8or large molecules


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IONI-ATION SOURCES


Baser 'esorption #onization

- ?ses pulsed laser

- !rovides selective ionization by choosing appropriate K

- Baser is focused on a solid surface to ionize material

=.amples of Basers

- #5 laserL CJlaser

- ?> laserL FdL1 (yttrium aluminum garnet)


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IONI-ATION SOURCES


Matri.-1ssisted Baser 'esorption #onization (M1B'#)

- Matri. disperses large amounts of energy absorbed by the laser

- Minimizes fragmentation of the molecule

- !ermits analysis of molecular 6eight over &,$,,, 'a

- ?sed for study of polymers$ proteins$ peptides


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IONI-ATION SOURCES


Matri.-1ssisted Baser 'esorption #onization (M1B'#)

Matri.- must be stable in vacuum and not react chemically

- must absorb strongly at laser K (6here analyte absorbs 6ea0ly)

=.amples

- #5 region L urea$ alcohols$ carbo.ylic acids

- ?> regionL G-hydro.ypicolinic acid$ *-chlorosalicylic acid


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IONI-ATION SOURCES


8ast 1tom Eombardment (81E)

- =mploys fast moving neutral inert gas atoms (1r) to ionizelarge molecules

- Sample is dissolved in glycerol and spread in a thin layer on

a metal probe

- !robe is then inserted into the mass spectrometer and a beam

of fast moving atoms probe the surface


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IONI-ATION SOURCES



- ?sed for analysis of surfactants and proteins (M9 < &,$,,,)

- 8or large and thermally unstable molecules

- Technique 6or0s 6ell at room temperature

- Simple and high sensitivity

- Sample can be recovered


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IONI-ATION SOURCES



- Modified technique is the continuous flo6 81E (C881E)

- Sample introduction is through a fused silica capillary tube

- Solvent flo6s continuously and sample is introduced by

continuous flo6 inIection

- 8or analysis of blood$ urine$ other body fluids$ 6aste 6ater


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IONI-ATION SOURCES

Ino%)anic MS Ioni.a!ion So(%c"

Solid Samples

- lo6 'ischarge (') and Spar0 sources

- 8or sputtering and ionizing species from solid surfaces

- !rimarily for atomic mass determination of elements

- ' has better S/F and able to sputter more material from sample


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IONI-ATION SOURCES

Ino%)anic MS Ioni.a!ion So(%c"

Biquid Samples

- #nductively coupled plasma (#C!)

- :as high ionization efficiency

- !rovides very simple mass spectra


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MASS ANALY-ERS

- 'ifferentiates ions according to their m/z

- 'ifferent designs are available

Scanning #nstruments- Jnly ions of a given m/z pass through the analyzer at

a given time

- Magnetic Sector Mass 1nalyzer

- uadrupole Mass 1nalyzer


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MASS ANALY-ERS

Simultaneous Transmission #nstruments

- 1llo6 transmission of all ions at the same time

- Time-of-flight (TJ8)

- #on Trap

- #on Cyclotron 5esonance Mass 1nalyzer

- 'ispersive Magnetic Mass 1nalyzer

Tandem Mass Spectrometer (MSn)

- T6o or more mass analyzers in sequence


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MAGNETIC SECTOR MASS ANALY-ER

- as phase molecules are ionized by a beam of high energy

electrons

- =lectrons may be eIected from molecules (ionization) or bonds

in molecules may rapture (fragmentation)

- #ons are then accelerated in a field (sector) at a voltage >

- Sector can have any ape. angle (,oand H,oare common)

- Most modern instruments combine both electric sector

and magnetic sector (double-focusing MS)


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- The electric sector acts as an energy filter

- m/z range is & @ &+,, for single-focusing and

*$,,, @ &,$,,, for double-focusing instruments

- =nergy of each ion % z>

- Ninetic energy depends on charge and voltage but not

on mass of ion

- #ons 6ith small masses must travel at a higher velocity

than ions 6ith larger masses


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- 8or single positively charged ions

m % mass of ionv % velocity of ion

z % charge of ion

> % accelerating voltage

- > changes as m varies such that O mvis constant

z>mv

& =&/-

m

-z>v

=

m

&Pv


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- #ons enter a curved section of a homogeneous magnetic

field E after acceleration

- #ons move in a circle 6ith radius r

- 1ttractive force on magnet % Ezv

- Centrifugal force on the ion % mv/r

- The t6o forces are equal if the ion follo6s the radius of

curvature of the magnet


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Substituting for v and rearranging gives

Ezvr

mv=

Ez

mvr =

>

rE

z

m =


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- 5adius of circular path depends on m/z if > and E are 0ept constant

- #ons 6ith different m/z travel in circles 6ith different radii

- Easis of separation by m/z

- #ons 6ith the right m/z reach the detector and others hit the

sides of the instrument and be lost

- 9hich m/z to reach the detector can be selected by varying > or E

- E is varied and > is 0ept constant in modern instruments


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TIME O LIGHT /TO0 ANALY-ER

- Ma0es use of a drift tube

- !ulses of ions are accelerated into the an evacuated drift tube

(free of field or e.ternal force)

- >elocity of an ion depends on m/z

(depends on mass if all ions have the same charge)

- Bighter ions move faster along the tube than heavier ions

- #ons are separated in the drift tube according to their velocities (v)


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TIME O LIGHT /TO0 ANALY-ER

- > % accelerating voltage

- #f B is the length of tube (typically &- m) and t is the flighttime of ion$ then v % B/t

- #mplies mass-to-charge ratio and flight time can be found from

- 1n ion mirror called a reflectron is used to increase resolution

&/

mz>v

=

B

>t

z

m=

z>

mBt =


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2UADRUPOLE MASS ANALY-ER

- Separates ions in an electric field (the quadrupole field)

- 8ield is varied 6ith time

- Jscillating radio frequency (58) voltage and a constant 'Cvoltage are used to create the field

- These are applied to four precisely machined parallel metal rods

- The result is an 1C potential superimposed on a 'C potential

- #on beam is directed a.ially bet6een the four rods


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- Jpposite pairs of rods are connected to opposite ends of a'C source

- #ons follo6 an oscillating (cor0scre6) path through the

quadrupole to the detector

- 8or a given ratio of 'C to 58 at a fi.ed frequency$ only ions of

a given m/z value 6ill pass through the quadrupole

- Jther ions 6ith different m/z values 6ill collide 6ith the

rods and be lost


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- The quadrupole acts as a filter so is often called the mass filter

- Sample must be ionized and in the gas phase

- m/z range is & @ &,,, 'a

- :as smaller range and lo6er resolution than magnetic sector

but faster

- #s the most common analyzer

- 5ugged$ ine.pensive$ and compact


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MS 3 MS /TANDEM MS0 INSTRUMENTS

- =mploys t6o or more stages of mass analyzers

- =.ample is t6o quadrupoles coupled in series

- 8irst analyzer selects ion (precursor ion) and second analyzerselects the fragments of the precursor ion

- ?sed to obtain more information about the structure of

fragment ions

- 8ragment ions may be dissociated into lighter fragment ions or

converted into heavier ions by reaction 6ith neutral molecule


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ION TRAP

- 1 device in 6hich gaseous ions are formed and/or storedfor periods of time

T6o commercial types

- uadrupole #on Trap (#T)

and

- #on-Cyclotron 5esonance Trap (#C5)


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ION TRAP

2(a+%(po, Ion T%ap /2IT0

- 1lso called !aul #on Trap

- ?ses a quadrupole field to separate ions

- 1 G' field is created using a ring-shaped electrode bet6een

t6o end cap electrodes

- 1 fi.ed frequency 58 voltage is applied to the ring electrode

- The end cap electrodes are either grounded or under

58 or 'C voltage


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ION TRAP

2(a+%(po, Ion T%ap /2IT0

- #ons are stored in trap by moving in traIectories bet6een

electrodes

- This is done by changing signs of electrodes to repel ions as

they approach the electrodes

- #ons of a given m/z pass through an opening to the detector

6hen the 58 of the ring electrode is changed

- m/z range is &, @ &,,, 'a


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ION TRAP

o(%i% T%an"#o%m IonC*c,o!%on R"onanc /TICR0

- 1lso called !enning #on Trap

- ?ses magnetic field to trap and store ions

- Consists of si. conducting plates arranged as a cube

- Cubic cell is about &,, mm on a side and is located inside

a strong magnetic field

- Sample is ionized by an electron beam


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ION TRAP

o(%i% T%an"#o%m IonC*c,o!%on R"onanc /TICR0

- The ions then move in circular orbits

- !ath is perpendicular to the applied field

- The operating frequency is called the cyclotron frequency


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DETECTORS

- Measure one m/z value at a time (single channel detectors)

- Multiple detectors are used for multiple ion detection

- :igh resolution magnetic sector instruments use multiple

detectors (called multicollectors)


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DETECTORS

E,c!%on M(,!ip,i% /EM0

- The most common detector in MS for ions

- Similar to !MT

- >ery sensitive and has fast response


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DETECTORS

a%a+a* C(p

- 1 metal or carbon cup serves to capture ions and store the charge

- Cup shape decreases loss of electrons

- Beast e.pensive detector for ions

- :as long response time


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DETECTORS

A%%a* D!c!o%"

- ?sed in TJ8 MS instruments

- =mploys a focal plane camera (8!C) consisting of an array ofG& 8araday Cup

- ?p to &* m/z values can be measured simultaneously

- =.hibits improved precision compared 6ith single channel

detectors

Documents

Chapter 9 Mass Spec 1